In prior art of integrated circuits, it is well known metallic ionic impurities can reduce the minority carrier lifetime in silicon bipolar devices. This is thought to be due to a relatively uniform distribution of the metallic contaminant in the semiconductor. A seemingly unrelated phenomenon observed in the prior art is the gettering of metallic ionic contaminants in regions of a semiconductor which are highly doped with boron. See, for example, J. E. Lawrence, Trans. AIME 242,484 (1968). Further, it has been observed; e.g. F. Barson, et al Fall Meeting of Electrochem. Society, October, 1969, Abstract 196, that ionized copper will diffuse across a silicon PN junction when a forward bias is applied. Two types of copper were identified: Interstitial in the p region and substitution copper in the n region of the device.
It has also been recognized that zinc is a conventional substitutional acceptor dopant, for forming p-n junctions in GaAs semiconductors. The zinc in the p type region will undergo conversion to an interstitial donor under certain doping conditions. This interstitial zinc has an extremely high diffusivity, causing the zinc to move under field-enhanced diffusion, across the p-n junction. This unwanted diffusion of zinc in GaAs has been observed to cause excessive leakage current in both forward and reverse biased directions, (see Longini, Solid State Electronics, 5, 127 (1962)).
Recently forward biased isolation-epitaxy junctions, (for example, the isolation-collector junction in an NPN transistor), have been observed to degrade through the diffusion of heavy metal ions, in particular copper, across the junction. This is caused principally by the high concentration of copper present in the isolation region since it is well established that this region acts as a "getter" for copper due to the high solubility of copper in degenerate silicon as well as the high density of dislocations plus lattice strain therein. The contaminant will undergo field-enhanced diffusion caused by a reduction in the built-in junction field and by the electric field in the p type and n type regions due to the ohmic field in these contiguous regions when the junction is forward biased.
FIG. 1a shows a cross-sectional view of a p type isolation region 2, typically of boron at a concentration of greater than 10.sup.19 atoms/cc, in a n type silicon epitaxial layer 4, of arsenic at a concentration of 10.sup.15 atoms/cc. A metallic contaminant such as copper is "gettered" in the isolation region 2.
FIG. 1b shows the concentration profile of FIG. 1a for boron, arsenic and the copper contaminant under equilibrium conditions. The copper concentration is approximately two orders of magnitude less than that of boron, see for example, (Hall, et al, Journal of Applied Physics, 35, 379 (1964)).
FIG. 1c shows the transient behavior for the redistribution of the metallic contaminant of FIG. 1b, for the case of a strong forward biased junction with a negligible built-in field remaining in the depletion region and no fields present in either the p or n regions. FIG. 1c illustrates the general transient behavior for ionized impurity diffusion out of the p region of the device when a strong forward bias condition is applied to the junction. At steady-state, since no internal electric field is present, a uniform impurity concentration will exist in all regions of the device.
FIG. 1d shows the transient behavior for the redistribution of the metallic contaminant of FIG. 1b, for a lower forward bias condition than that for FIG. 1c, where the built-in junction field, E.sub.J has a significant magnitude. The amount of diffusion of the contaminant across the junction is less than that in the strongly forward biased case shown in FIG. 1c, due to the compensating effect of E.sub.J. A general steady-state distribution of the contaminant as a function of several forward bias voltages is shown in FIG. 1e.
FIG. 1f illustrates a typical transient situation for a forward biased junction when the depletion region has a moderate retarding field, while the p region has an accelerating drift field. It should be noted that when steady-state conditions are attained, both regions still have concentration gradients since both drift and diffusion forces demand non-uniform concentrations. Under these conditions, more of the metallic contaminant will diffuse across the junction than that for FIG. 1d, but less than that for the strongly forward biased condition of FIG. 1c.